The purpose of XS is to convert from Perl calling semantics to C ones, convert the returned value to something perlish.

I don't have much XS experience, but it seems wrong to me to call a glue function from C. You'd do better to put a and b in another file.

/* my.xs */

#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
#include "my.h"

MODULE = MySOTester             PACKAGE = MySOTester

void 
a()

void
b()
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/* my.h */

extern void a();
extern void b();
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/* my.c */

#include "my.h"

#include <stdio.h>

void 
a()
{
    printf("Hello\n");
}

void
b()
{
    a();
    printf("World!\n");
}
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Not tested.